Method and Apparatus for Continuously Monitoring Interstitial Regions in Gasoline Storage Facilities and Pipelines

ABSTRACT

An underground storage system includes a primary containment unit and a secondary containment unit arranged to sealingly encompass the primary containment unit. The underground storage system further includes a leak detection system which is fluidly connected to the secondary containment system, and which is adapted to detect fluid leaks in the primary containment system and the secondary containment system.

RELATED APPLICATIONS

This application is a divisional of U.S. Ser. No. 11/253,341, filed Oct.19, 2005, which is a continuation-in-part of U.S. Pat. No. 7,051,579filed May 11, 2004.

TECHNICAL FIELD

This patent is generally directed to an apparatus and method forinterstitial monitoring, and more particularly to a system forcontinuously monitoring the pressure and vacuum levels within theinterstitial space of an underground storage tank system.

BACKGROUND

Current and proposed state and federal regulations require thatunderground storage tanks used for the storage of hazardous substancesmeet certain environmental safety requirements. In particular, theseenvironmental regulations require that the underground storage systemsinclude a primary containment unit and a secondary containment unit.Moreover, the primary and secondary containment units are required tocomply with the environmental standards that require underground storagetank systems to be product tight. The term “product tight,” for purposesof these environmental regulations, is generally defined as imperviousto the substance that is contained to prevent seepage of the substancefrom the primary containment unit. Moreover, for a tank to be producttight, the tank cannot be subject to physical or chemical deteriorationby the contained substance over the useful life of the tank. Further,these regulations require that owners or operators of an undergroundstorage tank system with a single-walled component located within 1,000feet of a public drinking water well implement a program of enhancedleak detection or monitoring.

One known method of monitoring leaks disclosed in U.S. Pat. No.6,489,894, entitled “Leak Detection Device for Double Wall PipelineSystems and Container Systems,” uses a leak detector with a vacuum pumpincluding a pressure-dependent switch and an alarm device to detectleaks in a double-walled pipeline or container system. The disclosedleak detector is adapted to simultaneously monitor several containersconnected to a collecting main and a vacuum pump by vacuum lines. Eachmonitored container incorporates a vacuum connector or valve to fluidlyconnect a control space to the leak detector. Each vacuum line has afirst liquid lock arranged at the vacuum connector to block liquid thathas leaked into the vacuum lines from a leaky container from penetratinginto the control spaces of the leak-free containers. A second liquidlock is arranged in the collecting main to prevent liquid from enteringthe vacuum pump. While this method can detect leaks within the controlspace of a container, it is a mechanically complex system requiring agreat deal of materials and set-up time.

Other methods of monitoring secondary or interstitial spaces are wellknown in the art and include continuous leak detection using bothpressure and brine solution monitoring techniques to determine thepresence or absence of leaks between the storage system and thesurrounding environment. However, to effectively calibrate all of theseknown methods and systems for operation, a great deal of set-up time andsystem knowledge is required. Specifically, to configure thesemonitoring systems for operation, the user must enter the volume of thesecondary or interstitial space to be monitored, which requires adetailed knowledge of the layout and the configuration of the doublewalled piping and containers used in the underground storage system.

SUMMARY

An underground storage system includes a primary containment unit and asecondary containment unit arranged to sealingly encompass the primarycontainment unit. The underground storage system further includes a leakdetection system that is fluidly connected to the secondary containmentsystem, and which is adapted to detect fluid leaks in the primarycontainment system and the secondary containment system.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the disclosed device, referenceshould be made to the following detailed description and accompanyingdrawings wherein:

FIG. 1 illustrates the basic components of an exemplary interstitialvacuum monitoring system;

FIG. 2 illustrates a flowchart detailing the operation of an exemplaryauto-learn routine;

FIG. 3 illustrates an exemplary interstitial vacuum curve; and

FIG. 4 illustrates a flowchart detailing the operation of an exemplarymonitoring routine.

DETAILED DESCRIPTION

FIG. 1 illustrates an exemplary underground storage system 10 thatincludes an underground storage tank (UST) 12 constructed to securelycontain a liquid 20, such as gasoline, diesel fuel or other hydrocarbon.The UST 12 is a double walled storage tank constructed with an outerwall 14, and an inner wall 16 separated to define an interstitial space18. In this manner, the UST 12 is divided into a primary containmentunit and a secondary containment unit to provide the underground storagesystem 10 with redundant leak protection.

A submersible turbine pump (STP) 22, such as, for example, the STP modelnumber STP-75-VL2-7 manufactured by FE PETRO, INC.(now Franklin FuelingSystems, Inc.), provides a means of pumping the liquid 20 to a dispenser24. The STP 22 may fixedly or removably mount to the UST 12 to positionan input nozzle 22 a below the surface of the liquid 20. The inputnozzle 22 a, in turn, provides a fluid path for pumping the liquid 20within the primary containment unit to the dispenser 24.

A pump manifold 26, which can be an integral component of the STP 22 ora separate component fixedly attached thereto, controls the distributionof the pumped liquid 20 to the dispenser 24. The pump manifold 26includes a siphon port 28 adapted to fluidly connect the interstitialspace 18 (e.g., secondary containment unit) to the vacuum generated bythe STP 22. Thus, when the STP 22 is active (e.g., producing a vacuum)the siphon port 28 provides a vacuum path to the interstitial space 18to evacuate the fluid contained therein. A control valve 30 can isolatethe interstitial space 18 from the siphon port 28 to prevent a vacuumdrop when the STP 22 is inactive and exposed to atmospheric pressure viathe primary containment unit.

A vacuum sensor 32 fluidly communicates with the interstitial space 18and the siphon port 28 to sample and measure the vacuum levels therein.The vacuum sensor 32 may be a continuous analog sensor, a discretedigital sensor, a switch based sensor, or any other device configured tosample the vacuum level within the interstitial space 18. The vacuumsensor 32 may be isolated by the control valve 30 to prevent atmosphericpressure measurements (i.e., zero vacuum measurements) when the STP 22is inactive. However, when the STP 22 is active and generating a vacuum,the control valve 30 opens to provide a fluid connection between thevacuum sensor 32, the interstitial space 18 and the siphon port 28. Inthis manner, the vacuum sensor 32 samples and measures the change in thevacuum level within the interstitial space 18 generated by the STP 22.

Further, the vacuum sensor 32 can communicatively connect to a controlunit 34 having a processor 36 and a memory 38. The control unit 34 andthe memory 38 receive and store vacuum data, system information, alarmdata, etc., from the vacuum sensor 32 or any other controlled component.Communications between the control unit 34 and, for example, the vacuumsensor 32 and the control valve 30, may be implemented using any desiredcommunications link, such as a hardwired local area network, a wirelesscommunications link, a direct communications link, or a point-to-pointwired communication link.

The processor 36 may execute a control routine to direct the set-up andoperation of the underground storage system 10. In particular, thecontrol routine may be written in any process control programminglanguage or computer language such as C⁺⁺, Visual C⁺⁺, Visual Basic,machine language and may be compiled (if necessary) and stored in thememory 38. Generally, the control routine insures the integrity of theunderground storage system 10 by detecting unwanted leaks. Inparticular, the control routine may execute on the processor 36 toautomatically learn the vacuum characteristics of the interstitial space18. Further, the control routine may include additional subroutinesadapted to execute on the processor 36 to continuously monitor thevacuum level within the interstitial space 18 as a function of time.

A leak orifice valve 40 fluidly connects to the control valve 30, thevacuum sensor 32, and a leak orifice 42, to provide a vacuum pathbetween the interstitial space 18. The leak orifice valve 40 and theleak orifice 42 can define a removable assembly adapted to disconnectfrom the interstitial space 18 when no longer required for the set-upand operation of the underground storage system 10. The leak orificevalve 40 allows for the automatic or manual creation of a calibrated orcontrolled leak between the interstitial space 18 and atmosphericpressure beyond the leak orifice 42. Such a controlled leak results in adecrease in the vacuum level within the interstitial space.

The vacuum sensor 32 can, in turn, measure the decreasing vacuum leveland communicate the vacuum level data to the control routine executingwithin the control unit 34 via the communications link. The controlroutine can, in turn, manipulate the vacuum level data to establish oneor more vacuum characteristics of the interstitial space 18. Inparticular, the control routine may determine a negative vacuum levelrate of change based on the decreasing vacuum level data caused by theintroduction of the controlled leak into the secondary containment unit.It will be understood that other vacuum characteristics, such as, forexample, a positive vacuum level rate of change, or the time to totalinterstitial space evacuation can be additionally or alternativelyestablished based on the vacuum level data.

The UST 12 can connect to other components of the underground storagesystem 10. In particular, the interstitial space 18 can fluidly connectto a secondary interstitial space 48 of a dispenser pipe 46 via aplurality of vacuum ports 44-44 b. In operation, the double-walleddispenser pipe 46 can provide the fluid connection between the liquid 20stored within UST 12 and the dispenser 24. Thus, the entire undergroundstorage system 10, including the UST 12 and the dispenser pipe 46, isdouble-walled and product tight against penetrations and corrosion thatmay be experienced during normal operations.

FIG. 2 illustrates a generalized operations flowchart of anauto-calibrating or auto-learn subroutine 50 adapted to learn the vacuumcharacteristics of the interstitial space 18. The auto-learn subroutine50 determines and stores the vacuum characteristics based, in part, onmeasured changes in the vacuum level as a function of time. Theauto-learn subroutine 50 learns the vacuum characteristics without theneed to determine or calculate the overall volume of the interstitialspace 18, the vacuum capacity of the STP 22, the sensitivity of thevacuum sensor 32, etc. In this manner, the auto-learn routine 50provides a fast and efficient means of calibrating and monitoring theinterstitial space 18 of any known or unknown volume or complexity. Itwill be understood that the auto-learn routine 50 can act as a standalone routine independent of the control routine or other subroutines.However, the auto-learn routine 50 can integrate with the controlroutine to satisfy the calibration requirements of the undergroundstorage system 10.

The auto-learn routine 50 can execute whenever a predetermined criteriahas been satisfied. In particular, the auto-learn routine 50 can executemanually as part of a regularly scheduled maintenance procedure, orautomatically in response to a change in the configuration of theunderground storage system 10, as part of the initial set-up andconfiguration of the underground storage system 10, or to compensate fora change in vacuum level over time.

A block 52 loads the stored initial settings and default conditionsrequired to execute the auto-learn routine 50 from the memory 38 (seeFIG. 1). These initial settings and default conditions can include,among other things, a maximum desired vacuum level P_(max), a minimumallowable vacuum level P_(min), closing the control valve 30, andcalibrating the vacuum sensor 32.

While the maximum desired vacuum level can be set to virtually anyvalue, empirical testing indicates that a vacuum level of approximately10 in. Hg (254 mm Hg), which represents an achievable vacuum level thatis easily distinguishable from atmospheric pressure, may be desirable.Similarly, the minimum acceptable vacuum level may be set to, forexample, 2 in. Hg (50.8 mm Hg). Typically, the minimum vacuum levelP_(min) provides a lower boundary or threshold to identify when thecurrent vacuum level P_(meas) within the interstitial space 18 isdecreasing towards atmospheric pressure (i.e., approx 0 in. Hg or zerovacuum).

A block 54 causes the vacuum sensor 32 to sample and measure the currentvacuum level P_(meas) within the interstitial space 18. Typically, thevacuum sensor 32 samples the current vacuum level P_(meas) at regulartime intervals At throughout the operation of the auto-learn routine 50.The memory 38 can store the vacuum level data representing the currentvacuum level P_(max) in a historical database as a stored vacuum levelP_(stored). The stored vacuum level P_(stored) can be permanentlyarchived in the historical database (i.e., saved in the database) or canbe temporarily stored for use in calculations, analysis, etc. andsubsequently erased or overwritten as new data is sampled and stored.

A block 56 compares the current vacuum level P_(meas) to atmosphericpressure (i.e., zero vacuum) to establish a vacuum baseline prior to theexecution of the remaining steps within the auto-learn routine 50. Upondetection of a vacuum in the interstitial space 18, a block 58 causesthe control valve 30 and the leak orifice valve 40 to open and vent thedetected vacuum to the atmosphere. A block 60 causes the vacuum sensor32 to sample the current vacuum level P_(meas) until atmosphericpressure is detected. When the vacuum sensor 32 detects atmosphericpressure, a block 62 closes the control valve 30 and the leak orificevalve 40 to seal and isolate the interstitial space 18 in preparationfor the execution of an evacuation procedure portion of the auto-learnroutine 50.

A block 64 initiates the evacuation procedure and the auto-learn routine50 begins to learn the vacuum level data required for generation of an“up curve” (an example of which is shown in FIG. 3 as the line 102). Inparticular, the block 64 activates the STP 22, which, in turn, begins toevacuate the interstitial space 18 via the siphon port 28. A block 66opens the control valve 30 to establish fluid communications between theSTP 22, the interstitial space 18, and the vacuum sensor 32. Typically,the control valve 30 opens after a delay period equal to the amount oftime required for the vacuum sensor 32 to detect the vacuum generated bythe STP 22. It will be understood that the delay period associated withthe vacuum sensor 32 may further depend on factors, such as thesensitivity of the vacuum sensor 32, the vacuum capacity of the STP 22,and the overall volume of the interstitial space 18.

A block 68 causes the vacuum sensor 32 to sample and measure the currentvacuum level P_(meas) within the interstitial space 18 at the timeinterval Δt. A block 70 causes the processor 36 to set the stored vacuumlevel P_(stored) equal to the current vacuum level P_(meas), and storethe resulting stored vacuum level P_(stored) in the historical databaseestablished within the memory 38. At this point, the evacuation or upcurve vacuum level rate of change within interstitial space 18 can becalculated based on the difference between the current vacuum level andthe stored vacuum level over a fixed or known time interval. Anevacuation rate of change ΔP_(evac) can be mathematically described bythe formula:${\Delta\quad P_{evac}} = \frac{P_{meas} - P_{stored}}{\Delta\quad t}$

The evacuation rate of change ΔP_(evac) describes the positive orincreasing slope of the evacuation curve representative of an increasein the vacuum level within the interstitial space 18. Alternatively, byplotting the current vacuum level P_(meas) values, and the stored vacuumlevel P_(stored) sampled during the operation of the auto-learnsubroutine 50 as functions of time the evacuation curve can beconstructed.

A block 72 compares the current vacuum level P_(meas) to a maximumdesired vacuum level P_(max). If the current vacuum level is less thanthe maximum desired vacuum level, the auto-learn routine 50 enters aloop 74 and continues to sample and store the current vacuum levelP_(meas) until the maximum desired vacuum level is achieved. However,when the block 72 detects that the current vacuum level exceeds themaximum desired vacuum level, a block 76 closes the control valve 30.

Subsequently, a block 78 deactivates the STP 22 and the evacuationprocedure concludes. At this point, the interstitial space 18 is sealedand isolated by the control valve 30, and the current vacuum levelP_(meas) remains substantially constant at the maximum desired vacuumlevel P_(max).

A block 80 causes the vacuum sensor 32 to sample and measure the currentvacuum level P_(meas) within the sealed interstitial space 18 at eachtime interval Δt. The current vacuum level P_(meas) is expected toremain at the maximum desired vacuum P_(max) level for a fixed number oftime intervals. Further, the memory 38 may store the current vacuumlevel P_(meas), which equals the maximum desired vacuum P_(max), in thememory 38 as the stored vacuum level P_(stored). At this point, thevacuum level rate of change within interstitial space 18 issubstantially zero. In other words, the vacuum level within the sealedinterstitial space is constant. A positive or negative change in thevacuum level during this time interval represents an anomaly, such as aleak, that will trigger an alarm. A maximum vacuum rate of changeΔP_(max) can be mathematically described by the formula:${\Delta\quad P_{\max}} = {\frac{P_{meas} - P_{stored}}{\Delta\quad t} = 0}$

The maximum vacuum rate of vacuum rate of change ΔP_(max) represents thezero-slope line corresponding to the maximum desired vacuum levelP_(max). It will be understood that determination of the maximum vacuumrate of change ΔP_(max) is an optional calculation that may be carriedout by the control unit 34.

A block 82 initiates the decay procedure and the auto-learn routine 50begins to learn the vacuum level data required to generate the “down” or“decay curve” (an example of which is shown in FIG. 3 as the line 106).In particular, the leak orifice valve 40 opens in response to a commandissued by the control routine executing within the control unit 34. Inoperation, the leak orifice valve 40, which may be a manual valve thatrequires operator intervention to open, provides a fluid path betweenthe current vacuum level of P_(meas) within the interstitial space 18and the zero vacuum level of the atmosphere. In other words, the leakorifice valve 40 provides an equalization path between the high vacuumlevel within the interstitial space 18 and the zero vacuum level ofatmospheric pressure. The decrease in the current vacuum level P_(meas)within the interstitial space 18 caused by the controlled leak providesa method for characterizing the performance of the secondary containmentunit in the presence of an actual, uncontrolled leak.

A block 84 causes the vacuum sensor 32 to sample and measure thedecreasing current vacuum level P_(meas) within the interstitial space18 at each of the time intervals Δt. A block 86 instructs the processor36 to store the deceasing current vacuum level P_(meas) in the memory 38as the stored vacuum level P_(stored). At this point, the decay or downcurve vacuum level rate of change within interstitial space 18 can becalculated based on the difference between the stored vacuum levelP_(stored) and the current vacuum level P_(meas) over a fixed timeinterval Δt. A decay rate of change ΔP_(decay) can be mathematicallydescribed by the formula:${\Delta\quad P_{decay}} = \frac{P_{stored} - P_{meas}}{\Delta\quad t}$

The decay rate of change ΔP_(decay) represents the negative slope of thedecay curve, which is the line defined by the decreasing current vacuumlevel P_(meas) values measured by the vacuum sensor 32 during the decayprocedure of the auto-learn routine 50.

A block 88 compares the current vacuum level P_(meas) to a minimumdesired vacuum level P_(min). It will be understood that the minimumdesired vacuum level P_(min) could be set to zero vacuum (i.e.atmospheric pressure) but will typically be set higher to reduce theoverall setup time for the system. In other words, the closer toatmospheric pressure that the minimum desired vacuum level P_(min) isset, the longer the interstitial space 18 takes to equalize. If thecurrent vacuum level P_(meas) is greater than the minimum desired vacuumlevel P_(min), the auto-learn routine 50 enters a loop 90 and continuesto sample and store the current vacuum level P_(meas) until the vacuumsensor 32 detects the minimum desired vacuum level P_(min) within theinterstitial space 18. However, if, at the block 88, the current vacuumlevel P_(meas) is less than the minimum desired vacuum level P_(min), ablock 92 cause the control valve 30 to close. At this point, the decayprocedure of the auto-learn routine 50 concludes and the learned ratesof change ΔP_(evac) and ΔP_(decay) can be combined to produce theoverall vacuum characteristics curve shown in FIG. 3.

FIG. 3 illustrates an exemplary overall vacuum characteristic curve 100embodying the learned rates of change ΔP_(evac), ΔP_(decay), and theoptionally derived ΔP_(max), measured and derived by the operation ofthe auto-calibrating routine 50. As previously indicated, the line 102represents the learned evacuation rate of change ΔP_(evac) derivedduring the auto-learn routine 50 and, in particular, illustrates apositive increase in the vacuum level of the interstitial space 18 as afunction of time. In physical terms, the line 102 represents the sealedinterstitial space 18 fluidly connected, via the control valve 30, tothe active STP 22. A maximum time T_(max) represents the amount of timerequired for the STP 22 to increase the current vacuum level within theinterstitial space 18 to the maximum desired vacuum level P_(max)

An upper range defined by the line 102 a and a lower range defined bythe line 102 b establish the allowable amount of vacuum level variationfrom the learned line 102 during the evacuation procedure. An alarmsubroutine can activate when the current vacuum level P_(meas) deviatesbeyond the acceptable limits established by the upper and lower rangesdefined by the lines 102 a and 102 b. For example, the alarm subroutinemay determine a leak exists within the interstitial space 18 when thecurrent vacuum level is determined to be outside of the upper and lowerranges defined by the lines 102 a and 102 b, or the maximum desiredvacuum P_(max) is not achieved by the time T_(max).

A line 104 represents the maximum desired vacuum level P_(max) and thelearned maximum vacuum rate of change ΔP_(max) equal to zero (i.e., thevacuum is constant). In physical terms, the line 104 represents theconstant current vacuum level measured when within the interstitialspace 18 is sealed and isolated from the STP 22, and the leak orificevalve 40. The isolated interstitial space 18 insures that the currentvacuum level P_(meas) remains virtually constant at P_(max) over thefixed number of time intervals.

As described previously, the line 106 represents the learned decay rateof change ΔP_(decay) derived during the auto-learn routine 50. The line106 illustrates a decrease in the measured vacuum level within theinterstitial space 18 as a function of time. In particular, the line 106corresponds to a system configuration wherein a controlled leak has beenintroduced into the underground storage system 10, and the currentvacuum level P_(meas) decreases as the vacuum within the interstitialspace 18 equalizes with atmospheric pressure (i.e., a vacuum level ofzero.)

As illustrated in FIG. 3, a permeation range 108 is defined by an upperline 108 a and a lower line 108 b sloping away from the line 106. Thepermeation range 108 represents the exemplary vacuum profile for thesealed interstitial space 18 as a function of time. In other words,during normal operations (e.g., steady state operations with no leaks orother variations) the current vacuum level P_(meas) is expected to bemeasured within the permeation range 108 defined by lines 108 a and 108b. The steady vacuum decay represented by the permeation range 108 isattributable to the natural permeation properties of the undergroundstorage system 10, rather than to a leak or other anomaly. However, ifthe current vacuum level P_(meas) or current vacuum level rate of changeΔP_(current) deviates from the range defined by the lines 108 a and 108b, (i.e., falls outside of the permeation range 108), then a leak orother anomaly is assumed to exist within the interstitial space 18 andthe alarm subroutine may activate.

FIG. 4 illustrates a flowchart detailing the operation of an exemplarymonitoring routine 120 employing the overall vacuum characteristic curve100. A block 122 causes the vacuum sensor 32 to sample and measure thecurrent vacuum level P_(meas) within the interstitial space 18. A block124 compares the current vacuum level P_(meas) to the minimum allowablevacuum level P_(min) (e.g., 2 in. Hg or zero vacuum). If the currentvacuum level P_(meas) is below minimum allowable vacuum level P_(min), ablock 126 activates the STP 22 which, in turn, begins to evacuate theinterstitial space 18 as generally indicated by the evacuation curve 102illustrated in FIG. 3.

A block 128 causes the control valve 30 to open, thereby establishingfluid communication between the STP 22, the interstitial space 18, andthe vacuum sensor 32. Typically, the control valve 30 opens after adelay period equal to the amount of time required for vacuum sensor 32to detect the vacuum generated by the STP 22. A block 130 instructs thevacuum sensor 32 to sample and measure the increasing current vacuumlevel P_(meas) within the interstitial space 18 at each of the timeintervals Δt.

A block 132 compares a current vacuum level rate of change P_(current)to the learned evacuation rate of change ΔP_(evac) determined during theauto-learn routine 50. It will be understood that the current vacuumlevel rate of change ΔP_(current) can be determined based on thedifference between the current vacuum level P_(meas) and the storedvacuum levels P_(stored) as a function of time. A current vacuum levelrate of change ΔP_(current) can be described by the formula:${\Delta\quad P_{current}} = \frac{P_{meas} - P_{stored}}{\Delta\quad t}$

If the current vacuum level rate of change ΔP_(current) is determined tobe less than the learned evacuation rate of change ΔP_(evac), a block134 may activate the alarm routine. However, if the current vacuum levelrate of change ΔP_(current) exceeds the learned evacuation rate ofchange ΔP_(evac), a block 136 instructs the processor 36 to store theincreasing current vacuum level P_(meas) in the memory 38 as the storedvacuum level P_(stored).

A block 138 compares the current vacuum level P_(meas) to a maximumdesired vacuum level P_(max). If the current vacuum level P_(meas) isless than the maximum desired vacuum level P_(max), the monitoringroutine 120 enters a loop 140 and continues to sample and store thecurrent vacuum level P_(meas) until the maximum desired vacuum levelP_(max) is detected. However, if the current vacuum level P_(meas)exceeds the maximum desired vacuum level P_(max), a block 142 causes thecontrol valve 30 to close.

A block 144 deactivates the STP 22 upon completion of the evacuation ofthe now-sealed interstitial space 18. Thus, the monitoring routine 120has recharged the vacuum level within the interstitial space 18. Inoperation, the evacuation or increase in the vacuum level of theinterstitial space 18 proceeds along the learned evacuation vacuum curve102, and the monitoring routine 120 continually verifies that thecurrent vacuum level P_(meas) remains within the predefined rangedefined by the lines 102 a and 102 b. Simultaneously, the time requiredto recharge the interstitial space 18 to the maximum desired vacuumlevel P_(max) can be compared to the maximum time T_(max). If thecurrent recharge time exceeds the maximum time T_(max), a leak or otheranomaly is assumed to exist and the alarm routine 134 activates.

A block 146 restarts the monitoring routine 120 so that the vacuumsensor 32 samples and measures the current vacuum level P_(meas) at theblock 122. At the block 124, the recently recharged current vacuum levelP_(meas) is compared to the minimum allowable vacuum level P_(min)(e.g., 2 in. Hg or zero vacuum). Because the recently recharged currentvacuum level P_(meas) is greater than the minimum allowable vacuum levelP_(min), a block 148 compares the current vacuum level rate of changeP_(current) to the learned decay rate of change ΔP_(decay) determinedduring the auto-learn routine 50.

As previously discussed, the interstitial space 18 is sealed and themonitoring routine 120 measures the current vacuum level P_(meas) todetermine if the decrease in the current vacuum level P_(meas) isattributable to the natural permeation properties of the undergroundstorage system 10 or to a leak. Furthermore, the comparison between thelearned vacuum curve and the current vacuum level P_(meas) can be basedon the difference between the decay rate of change ΔP_(decay) and thecurrent rate of change ΔP_(current) or simply based on the differencebetween the current vacuum level P_(meas) and the learned vacuum curveitself.

A block 150 instructs the processor 36 to store the current vacuum levelP_(meas) in the memory 38 as the stored vacuum level P_(stored). At thispoint, the monitoring routine 120 enters a loop 152 and continues tosample and store the current vacuum level P_(meas) until the minimumallowable vacuum level P_(min) is detected, at which time the STP 22activates to evacuate the interstitial space 18.

Similarly, monitoring of the vacuum level during evacuation can also beused to monitor for problems. The system uses the learned evacuationrate of change ΔP_(evac), or upcurve, as illustrated as line 102 (FIG.3) to determine if any liquid ingress has occurred inside the secondarycontainment. This is accomplished by comparing the learned up-curve inmemory to the currently measured up-curve. If the slope of the currentmeasured up-curve is greater than the slope of the learned up-curve by athreshold factor exceeding that defined by line 102 a (FIG. 3), (i.e.,it took sufficiently less time to evacuate the containment space thanwhat was originally learned), then liquid is suspected to have enteredthe secondary containment. This is due to the fact that the liquidingress has effectively reduced the containment area available to thevacuum. Additionally, if the slope of the current measured up-curve isless than the slope of the learned up-curve by a threshold factorexceeding that defined by line 102 b (FIG. 3), (i.e. it tooksufficiently longer to evacuate the containment space than what wasoriginally learned), then it is possible that there is a leak in thevacuum suction line, permitting fluid to enter. In either case (acurrently measured slope sufficiently greater than or sufficiently lessthan the learned slope) will trigger an alarm. In this way, a physicalliquid collection chamber and liquid sensor is not required, reducingthe cost and complexity of the system.

While the embodiments described herein have been directed to vacuumlevel measurements and analysis, it will be understood that anoverpressure within the interstitial space 18 may be employed to providea pressure gradient suitable for measurement by the auto-learn routine50 and monitoring by the monitoring routine 120. Further, it will beunderstood that the current vacuum level P_(meas) and the calculatedrates of change can be determined in a manual fashion. For instance,manual instructions may direct the control unit 34 to sample and storethe current vacuum level P_(meas) within the interstitial space 18.Moreover, an operator may employ the rate of change formulas andconcepts discussed above in conjunction with the stored vacuum levelsP_(stored) to manually calculate the desired rates of change.

Although certain embodiments have been described in accordance with theteachings of the present disclosure, the scope and coverage of thispatent is not limited thereto. To the contrary, this patent is intendedto cover all embodiments of the teachings of the disclosure that fairlyfall within the scope of the permissible equivalents.

1. An underground storage system comprising: a primary containment unit;a secondary containment unit arranged to sealingly encompass the primarycontainment unit; a vacuum system for periodically applying a vacuum tothe secondary containment unit; sensor circuitry for determining a rateof change of vacuum pressure in the secondary containment unit as thevacuum system applies the vacuum; a leak detection system, wherein theleak detection system is fluidly connected to the secondary containmentunit and adapted to learn a vacuum rate of change of the secondarycontainment unit when the secondary containment unit is void of liquid,as the vacuum system applies the vacuum; and wherein the leak detectionsystem is adapted to detect a presence of liquid in the secondarycontainment unit if the determined rate of change of vacuum pressure inthe secondary containment unit exceeds the learned vacuum rate of changeof the secondary containment system by a threshold amount.
 2. A methodof monitoring for liquid in a secondary containment unit, the methodcomprising: generating a first vacuum level rate of change in responseto a first evacuation of the secondary containment unit when thesecondary containment unit is void of liquid; determining a secondvacuum level rate of change within the secondary containment unit inresponse to a second evacuation of the secondary containment unit;comparing the first vacuum level rate of change to the second vacuumlevel rate of change; and activating an alarm if the second vacuum levelrate of change exceeds the first vacuum level rate of change beyond athreshold amount.